1. Field of the Invention
The present invention relates to hydraulic intensifier units for use in oil well stimulation such as is employed for fracturing geological formations adjacent deep well bores and for erosion drilling and the like.
2. Prior Art
Apparatus of the type to which the present invention relates are disclosed, for example, in U.S. Pat. No. 3,773,438 and 3,981,622. The basic principle of operation of such devices is to use a pair of hydraulic cylinder motors each associated with a ram to which is supplied the fluid to be pumped into a well at high pressures for fracturing the substrata. Each of the hydraulic cylinder motors is provided with a high volume of hydraulic fluid through a controlled valving arrangement which sequentially drives the respective pistons of the hydraulic cylinder motors to their extended positions, which in turn, through piston rods, forces the rams forward to expel a fluid into the well.
In addition, these devices are provided with a precompression valving arrangement which pressurizes the working face side of the pistons of the cylinder motors when they are in the retracted position, prior to their being forced forward by the main source of hydraulic fluid. This precompression assists in maintaining uninterrupted pressure on the fluid being forced into the well, by maintaining a smooth transition from pressure being applied alternately by the two cylinder motors. As the piston of one cylinder motor is being moved forward in its working stroke the opposite piston is being returned to its retracted position and the ram is filled with fluid to be pumped into the well. In the retracted position a small volume remains on the working face side of the hydraulic cylinder motor which is then precompressed in order to precompress the fluid in the ram for smooth transition from one ram to the other to maintain constant pressure on the fluid being pumped into the well, as is mentioned above.
The sequential operation of both rams is controlled through pneumatic and hydraulic fluid control systems as described in each of the above referred to patents. In addition, the two patents referred to above describe two different means of returning the pistons in the cylinder motors to their initial starting positions. The first patent discloses a pair of return hydraulic cylinder motors associated with each of the power rams which act upon the rams after completion of their power stroke to return the rams to their retracted position. The second referred to patent utilizes compressed air on the return face side of each piston of the hydraulic cylinder motors which, upon completion of the power stroke of the respective piston, returns the piston to its initial starting position.
The primary operating limitation on the cycling speed of the intensifier unit, which necessarily limits the amount of fluid which can be pumped into a well, is the volume of hydraulic fluid which in a given time, can be supplied by the hydraulic fluid source to the working faces of the hydraulic cylinder motors. The source of fluid supply is generally a series of pumps operated by relatively high horsepower diesel engines. Thus, when the pumps are operating at maximum throughput the maximum rate of flow of hydraulic fluid to the cylinder motors is attained which in turn limits the speed the pistons and their respective rams can be moved forward and thus limits the volume of fluid per rate of time that can be pumped.
In the past, it has been possible to adjust the cycle of operation of the intensifier unit by changing the size of the pistons in the rams and by inserting sleeves in the rams to reduce the volume of fluid pumped into the well on each stroke while increasing the pressure.
It is generally advantageous to pump at as high a fluid flow rate as possible in order to fill the cavity of the well in the shortest time possible. However, with limitations on the pressure of the hydraulic fluid due to the practical limitations of the source of supply, as mentioned above, the pressure which can be generated at maximum fluid pumping rate may not be sufficient to fracture the substrata. It then becomes necessary to change the pistons and sleeves in the rams to decrease the area of the piston and the volume in the ram so as to increase the pressure. This also, however, decreases the rate of fluid flow from the rams. In the past, it has been necessary to physically change the pistons in the rams and insert or remove sleeves which increases or decreases the volumes in the rams in order to achieve this dual purpose of maximum rate of fluid flow and high pressure needed to fracture some geological formations.